Add JWK.getKeyStore method

[Connect2id OSS]

For JWKs that have been loaded from a file or PCKS#11 KeyStore, so that we can refer to the key store after the key has been loaded.

Useful with JWK sets where some keys come from an HSM, and others are file based, so we can choose the correct provider later for signing / decryption.

[Connect2id OSS]

Done: 6aef9a8